A wide variety of absorbent structures for use in disposable absorbent articles are known in the art. Common commercial absorbent articles include diapers, adult incontinence products, catamenials and bandages. These products are provided with various functional components for receiving, absorbing and retaining fluids.
Disposable absorbent articles, such as diapers, feminine hygiene products, adult incontinence devices and the like have found widespread acceptance. To function efficiently, such absorbent articles must quickly absorb body fluids, distribute those fluids within and throughout the absorbent article and be capable of retaining those body fluids with sufficient energy to dry the surface when placed under loads. In addition, the absorbent article need be sufficiently soft and flexible so as to comfortably conform to body surfaces and provide close fit for lower leakage.
While the design of individual absorbent articles varies depending upon use, there are certain elements or components common to such articles. The absorbent article contains a liquid pervious top sheet or facing layer, which facing layer is designed to be in contact with a body surface. The facing layer is made of a material that allows for the unimpeded transfer of fluid from the body into the core of the article. The facing layer should not absorb fluid per se and, thus, should remain dry. The article further contains a liquid impervious back sheet or backing layer disposed on the outer surface of the article and which layer is designed to prevent the leakage of fluid out of the article.
Disposed between the facing layer and backing layer is an absorbent member referred to in the art as an absorbent core. The function of the absorbent core is to absorb and retain body fluids entering the absorbent article through the facing layer. Because the origin of body fluids is localized, it is necessary to provide a means for distributing fluid throughout the dimensions of the absorbent core to make full use of all the available absorbent material. This is typically accomplished either by providing a distribution member disposed between the facing layer and absorbent core and/or altering the composition of the absorbent core per se.
Fluid can be distributed to different portions of the absorbent core by means of a transfer or acquisition layer disposed between the facing layer and core. Because of the proximity of such an acquisition layer to the body surface of the wearer, the acquisition layer should not be formed from material that retains large amounts of fluid. The purpose of the acquisition layer is to provide for rapid transfer and distribution of fluid to the absorbent core while minimizing spread of the fluid in this layer.
The absorbent core is typically formulated of a cellulosic wood fiber matrix or pulp, which pulp is capable of absorbing large quantities of fluid. Absorbent cores can be designed in a variety of ways to enhance fluid absorption and retention properties. By way of example, the fluid retention characteristics of absorbent cores can be greatly enhanced by disposing superabsorbent materials amongst fibers of the wood pulp. Superabsorbent materials are well known in the art as substantially water-insoluble, absorbent polymeric compositions that are capable of absorbing large amounts of fluid in relation to their weight and forming hydrogels upon such absorption. Absorbentarticles containing blends or mixtures of pulp and superabsorbents are known in the art.
The distribution of superabsorbents within an absorbent core can be uniform or non-uniform. By way of example, that portion of an absorbent core proximate to the backing layer (farthest away from the wearer) can be formulated to contain higher levels of superabsorbent than those portions of the core proximate the facing or acquisition layer. By way of further example, that portion of the core closest to the site of fluid entry (e.g., acquisition zone) can be formulated to transport (wick) fluid into surrounding portions of the core (e.g., storage zone).
As consumer demand for less expensive and less bulky disposable absorbent products increases, manufacturers continue to seek effective ways to reduce size and cost without sacrificing the quality of the fluid transport properties or structural integrity of the products during use. Preferably, the disposable products should be soft, thin and absorbent. Unfortunately, when softness and thinness characteristics are improved through the use of additives known in the art, absorbency is sacrificed. Therefore, there is a need in the industry for manufacturing processes which produce a soft and thin material while maintaining the desired level of absorbency.
Absorbent materials, such as absorbent cores may be made from pulp in the form of sheets or board. The pulp is defiberized in order to manufacture an absorbent material. Therefore, additives which improve the characteristics of the absorbent end product may be added 1) to the pulp, 2) during the manufacture of pulp board, or 3) during manufacture of the absorbent core. Conventionally, for the manufacture of absorbent materials, softness is achieved by the addition of debonders to the pulp, since the softness of a pulp product is greatly influenced by the degree to which the constituent wood pulp is debonded, i.e., the extent to which hydrogen bonds within the wood pulp are broken. For example. debonder is added to the pulp while the pulp is in the holding chests, prior to deposition of the pulp slurry on the Fourdrinier wire. The result is softer pulps or pulp products, which typically have decreased hydrogen bonding.
Wood pulp softness can be expressed in terms of properties such as Mullen strength (the strength of pulp or a pulp product, measured in kilopascals (kPa), defined in greater detail below), and Kamas energy (the energy required to convert a given amount of pulp or pulp product to a fluff material, measured in watt hours per kilogram (Wh/kg), defined in greater detail below). Lower values of Mullen strength and Kamas energy correlate to softer, increasingly debonded, pulp.
Many industrial pulp applications involve the conversion of pulp to fluff pulp by mechanical means. The efficient mechanical fluffing of wood pulp requires a pulp (stiff) that will debond to a desirable degree with minimum energy input and little mechanical fiber damage. Such pulp must have the proper bulk and degree of inter-fiber bonding. A hard pulp sheet will increase the energy needed to create fluff pulp and will therefore lead to increased fiber damage, while an unduly soft pulp sheet will lead to pull-out of large pieces of pulp, causing poor fluffing.
Currently, cationic compounds are used in the manufacture of wood pulp products such as sanitary papers to yield a product which has a soft hand feel. This is accomplished through the lubricating nature of the substantive softening molecule; less extensive inter-fiber bonding leading to greater bulk and the plasticizing effect of these additives.
There are several cationic chemical materials known for use to soften pulp to produce a fluff or debonded pulp. These cationic materials are quaternary ammonium compounds, as disclosed in U.S. Pat. Nos. 3,554,862; 3,677,886; 3,809,604; 4,144,122 and 4,432,833 among others.
Nonionic agents are also used to a limited extent to debond pulp in the paper industry (BEROCELL 587, available from Eka Nobel) but even they cause adverse affects on absorbency. It is believed that this effect is due to the presence of long hydrophobic side chains.
Water-soluble polyhydroxy alcohols have been disclosed as softening agents in U.S. Pat. No. 2,249,118. Non-ionic compounds such as fatty acid esters in combination with cationic retention agents have been disclosed in U.S. Pat. No. 4,303,471 to obtain good disintegration properties for pulp.
However, pulp softening and debonding, when accomplished by the conventional treatments described above, will result in a material that is less absorbent.
As an alternative to the use of additives, materials may be specifically engineered to obtained the desired characteristics. In addition to blending pulp with superabsorbent material, a variety of other means for improving the characteristics of pulp have been described.
One particular desirable character is the stiffness of the absorbent core. As the disposable hygiene products industry moves towards ultrathin products, stiffness of the absorbent core has become a critical issue. The use of mercerized fibers to reduce the stiffness (or improve the softness) of absorbent cores has been disclosed in U.S. Pat. No. 5,866,242. However these fibers arc expensive when compared to non-mercerized fibers.
Debonding agents such as quaternary ammonium compounds can be used to produce soft pulp sheets and absorbent cores. However, as previously mentioned, the use of debonders result in a substantial negative impact on absorbency.
There have been numerous attempts by the manufacturers of absorbent materials to produce core materials which arc highly absorbent, strong, and soft U.S. Pat. No. 4,610,678 discloses an air-laid material containing hydrophilic fibers and superabsorbent material, wherein the material is air-laid in a dry state and compacted without the use of any added binding agents. Such material, however, has low integrity and suffers from shake-out or loss of substantial amounts of superabsorbent material. U.S. Pat. No. 5,516,569 discloses that superabsorbent material shake-out can be reduced in air-laid absorbents by adding significant amounts of water to material during the air-laying process. The resultant material, however, is stiff of low density and has a high water content (&gt;about 15 weight percent). The high stiffness can be reduced using embossing which requires an additional processing step in the manufacturing process, but embossing has been shown to have a negative impact on the tensile strength of the core. Thus, use of the above steps to produce soft absorbent cores results in substantial increases in the cost of the core.
For example, soft absorbent materials are disclosed in U.S. Pat. Nos. 5,866,242 and 5,916,670. However, the processes disclosed therein may be expensive, so it would be desirable to use an additive which would provide a lower cost alternative, if such additive could be used to treat pulps to provide beneficial properties to pulp, and thereafter to absorbent cores formed from treated pulps, without decreasing absorbency.
There continues to be a need in the art, therefore, for a material that satisfies the absorbency, strength and softness requirements needed for use as absorbent core in disposable absorbent articles and which simultaneously provides time and cost savings to both the pulp manufacturer and the manufacturer of the absorbent article.
Accordingly, it would be desirable to provide a method of treating pulp sheets to form fluff pulp with improved softness without sacrificing the absorbent properties of the pulp.